Animated methods do not trigger callback when speed is 0...
I happen to be working on a plug-in where the FX used to show elements is
configurable. I want the default behavior to just show an element with no
delay, but I noticed that if you specify the FX with a 0 speed than the
callback is never triggered.
I'm sure this is only ever going to be encountered when writing plug-ins,
but it definitely should trigger the callback.
I just filled a bug for this:
http://dev.jquery.com/ticket/2542
-Dan